Geographic and behavioral differences associated with sexually transmitted infection prevalence among Indian men who have sex with men in Chennai and Mumbai

India has one of the largest numbers of men who have sex with men (MSM) globally; however, geographic data on sexually transmitted infection (STI) prevalence and associations with sexual behavior are limited. Six-hundred and eight MSM in Chennai and Mumbai underwent screening for a behavioral trial and were assessed for bacterial STIs (syphilis, chlamydia, gonorrhea), HIV, and past-month self-reported condomless anal sex (CAS). Mumbai (37.8%) had a greater prevalence of any STI than Chennai (27.6%) (prevalence ratio [PR] = 1.37, 95% CI: 1.09, 1.73). This pattern also emerged for gonorrhea and chlamydia separately but not syphilis. Conversely, Mumbai MSM reported lower rates of CAS (mean = 2.2) compared to Chennai MSM (mean = 14.0) (mean difference = −11.8, 95% CI: −14.6, −9.1). The interaction of city by CAS on any STI prevalence (PR = 2.09, 95% CI: 1.45, 3.01, p